Is it permissible for a single Udhiyah to be offered on behalf of two brothers, one of whom lives separately from his parents and shares accommodation with his unmarried brother, while the other lives separately with his wife? And in this case, would the Udhiyah include all members of both families?

If the sacrificial animal is a camel or a cow, then it is permissible to share in it because one of them suffices for seven. However, if it is a sheep, then one sacrifice suffices for a man and his household who share with him in dwelling and expenses. A Muslim sacrifices on behalf of himself and those whose maintenance is his responsibility, and he does not share with anyone who is independent in dwelling or expenses. Therefore, based on this, you would sacrifice on behalf of yourself and your younger brother who lives with you, and you would not share with your married brother who resides in another house, nor with your parents.